a) Field of the Invention
This invention relates to a focus detecting device for use in cameras.
b) Description of the Prior Art
As an example of conventional focus detecting devices, an arrangement is made such that an image formed by a photographic lens is divided into two by a reimaging optical system, which are re-formed on light receiving element arrays (photoelectric converting element arrays), and the positional shift of the two images is detected, thereby allowing focus detection to be made. For such a device, various proposals have been made in the past as set forth, for example, in Japanese Patent Preliminary Publication Nos. Sho 55-118019, Sho 58-106511, and Sho 60-32012. Any of these proposed devices is adapted to use light receiving element arrays in a line for focus detection and has the following characteristics.
In a focus detecting (optical) system, the size of a focus detecting area is usually incompatible with focusing accuracy. That is, the light receiving element array is such that light receiving elements are generally arrayed at equal spaces and, when the space between two light receiving elements adjacent to each other is taken as one pitch, focusing accuracy is usually represented as a measure relative to one pitch. Now, when focusing accuracy is assumed to be 1/M (M is a constant) of one pitch and the amount of defocus per pitch at the image plane is taken as .alpha., a focusing accuracy .DELTA. at the image plane is defined as EQU .DELTA.=.+-.(1/M).alpha. (1)
Here, if the value .DELTA. is large, focusing accuracy will be poor, while if it is smaller, focusing accuracy will be improved.
When the number of light receiving elements of the light receiving element array is taken as N (constant), a detectable defocus area Z at the image plane is given by EQU .SIGMA.=.vertline..+-.N.alpha..vertline. (2)
Here, if the amount of defocus is increased, the focus detecting area .SIGMA. will be large but the focusing accuracy .DELTA. will be poor. Conversely, if the amount of defocus .alpha. is decreased, the focusing accuracy .DELTA. will be improved but the focus detecting area .SIGMA. will be diminished, and thus both are incompatible with each other. Hence, the use of the devices of the above mentioned prior art makes it impossible to satisfy the conditions of the focus detecting area and the focusing accuracy at once and to perform complete focus detection.
Further, the conventional device of the type is constructed so that light receiving element arrays are made in a line as a focus detecting system and the amount of defocus of the photographic lens is detected from contrast information only in one direction of an object corresponding to the light receiving element arrays, with the result that focus detection cannot be performed with respect to an object having no contrast in the directions in which the light receiving elements are arrayed.
In order to obviate these drawbacks, a focus detecting device provided with a plurality of focus detecting systems, which will be described below, is proposed. An example of such a device is shown in Japanese Patent Preliminary Publication No. Sho 63-88511. This focus detecting device is designed so that the extension of the focus detecting area and the improvement of the focusing accuracy are satisfied at once and focus detection can be effected irrespective of the directions of contrast of the object.
FIG. 1 shows a camera body, on the bottom of which a focus detecting device 1 is arranged. FIG. 2 shows an optical arrangement of the focus detecting device 1 including two focus detecting systems A and B perpendicular to each other. For these systems, FIG. 3 depicts the focus detecting system A, and FIG. 4 the focus detecting system B, perpendicular to the focus detecting system A, in a state where the system B is rotated 90.degree. about the optical axis with respect to FIG. 3. In the figures, for the focus detecting device 1, a condenser lens 5 is disposed behind and adjacent to a preset imaging plane 4 of a photographic lens 2, and an aperture stop 6 is disposed behind the condenser lens 5 as shown in FIG. 5, two pairs of apertures are punched in aperture stop 6 in directions perpendicular to each other, and two apertures having a space sufficient to ensure focusing accuracy for each pair. A separator lens 7 is disposed behind the aperture stop 6 and has two pairs of reimaging lenses perpendicular to each other, and as illustrated in FIG. 6 corresponding to individual apertures. Behind the separator lens 7, two pairs of light receiving element arrays 8 and 9 are disposed in directions normal to each other. As shown in FIG. 7, light receiving element arrays 8 and 9 are disposed at imaging positions of individual beams of light emerging from the separator lens 7. Individual light beams passing through the apertures of the aperture stop 6 are independent of one another. The two focus detecting systems A and B perpendicular to each other consist of a first focus detecting system A and a second focus detecting system B, respectively. The amounts of defocus to be detected in terms of the first and second focus detecting systems A and B are determined as follows: EQU D.sub.1 =(F.sub.W1 /.beta..sub.1) P.sub.1 ( 3) EQU D.sub.2 =(F.sub.W2 /.beta..sub.2) P.sub.2 ( 4)
where D.sub.1 is the amount of defocus to be detected in the first focus detecting system A, D.sub.2 is the amount of defocus to be detected in the second focus detecting system B, P.sub.1 is the amount of phase difference of the images on the light receiving element arrays 8 of the first focus detecting system A, P.sub.2 is the amount of phase difference of the images on the light receiving element arrays 9 of the second focus detecting system B, .beta..sub.1 is the imaging magnification of the first focus detecting system A, .beta..sub.2 is the imaging magnification of the second focus detecting system B, F.sub.W1 is the F number of the barycentric beam to be detected in the first focus detecting system A, and F.sub.W2 is the F number of the barycentric beam to be detected in the second focus detecting system B. Here, the term "barycentric beam" means the light beam defined by rays passing through the center of each aperture of the aperture stop.
In Equations (3) and (4), the amounts of defocus D.sub.1 and D.sub.2 are determined by properly setting the F numbers F.sub.W1 and F.sub.W2 and the magnifications .beta..sub.1 and .beta..sub.2 of the first and second focus detecting systems A and B. Then, in connection with Equations (1) and (2), for example, the first focus detecting system A can be constructed as an optical system which is somewhat low in focusing accuracy but large in detectable defocus area, while the second focus detecting system B as another optical system which is smaller in focus detecting area but higher in focus accuracy. Thus, the focus detecting device can be derived which satisfies the conditions of the extension of the focus detecting area and the improvement of the focusing accuracy at once. In this optical system, however, the positional relationships between the primary imaging plane 4, the condenser lens 5, the separator lens 7, and the light receiving element arrays 8 and 9 and the configuration of the condenser lens 5 are the same in the first and second focus detecting systems, because the imaging magnifications .beta..sub.1 and .beta..sub.2 must be equal in order to dispose the light receiving element arrays on the same plane. For this purpose, it is required that the difference between the F numbers F.sub.W1 and F.sub.W2 is increased to fully satisfy the conditions of the extension of the focus detecting area and the improvement of the focusing accuracy at once. If the first focus detecting system A has low contrast in the directions of the light receiving element arrays, focusing can be performed by the second focus detecting system B.
For at least one of the plural focus detecting systems, according to this suggestion, two beams leaving the photographic lens for entering a pair of light receiving element arrays 8, as shown in FIG. 8, are incident at different angles .theta..sub.B and .theta..sub.C on a quick-return mirror 10 constructed from a half mirror. The two beams are therefore subjected to different refracting actions, and the spectral transmittance of the half mirror varies with the incident angle, so that the characteristics of spectra incident on the sensors for the two beams are also different. This makes it impossible to effect accurate focus detection. On the contrary, Japanese Patent Preliminary Publication Nos. Sho 63-118112 and Sho 64-56407 each propose the method of correcting the different refracting actions attributable to the half mirror by designing the separator lens asymmetrically. Japanese Patent Preliminary Publication No. Hei 2-132407 discloses the technique of evaporating a dielectric multilayer film such that the spectral transmittance of the half mirror does not vary with the incident angle.
By the means mentioned above, the focus detecting device can be designed which satisfies the conditions of the extension of the focus detecting area and the improvement of the focusing accuracy at once. Further, the focus detecting device detectable, irrespective of the directions of contrast of the object, can be constructed. Provisions have also been made for the difference between the refracting actions and the variation of the spectral transmittance of the half mirror with the incident angle where two beams are incident at different angles on the quick-return mirror using the half mirror. The method of configuring the separator lens asymmetrically, however, requires correction for errors produced on assembling the optical system in order to cancel securely the different refracting actions caused by the quick-return mirror and the disadvantage in manufacture and assembly of the optical system. When the separator lens of plural focus detecting systems is integrally constructed in particular, this tendency becomes pronounced. In order that the spectral transmittance of the half mirror does not vary with the incident angle, it is necessary to be severe on the tolerance of thickness of the dielectric multilayer film. Nevertheless, the variation of the spectral transmittance with the incident angle cannot be made to vanish completely.
Another detecting device satisfying the condition of both the focus detecting area and the focusing accuracy is proposed by Japanese Patent Preliminary Publication No. Sho 63-264715. The optical system shown in FIG. 9 is directed to a focus detecting device in which two focus detecting systems are juxtaposed which is based on a TTL phase difference technique used in one embodiment of the proposed device. In this figure, a half mirror 11 for splitting an optical path and a reflecting mirror 11' are placed behind the condenser lens 5. On the optical path reflected from the half mirror 11 are arranged in order an aperture stop 12 having a pair of apertures Juxtaposed, normal to the plane of the figure, at a distance sufficient to ensure focusing accuracy; a pair of reimaging lenses 13 located behind the apertures; and light receiving element arrays 14 placed at imaging positions of beams of light passing through the reimaging lenses 13. On the optical path transmitted through the half mirror 11 and reflected from the mirror 11', on the other hand, are arranged in order an aperture stop 15 having a pair of apertures juxtaposed, normal to the plane of the figure, at a distance sufficient to ensure focusing accuracy; a pair of reimaging lenses 16 constructed integral with, but different in location from, the reimaging lenses 13; and light receiving element arrays 17 situated at imaging positions of beams of light passing through the reimaging lenses 16.
An optical system lying on the optical path reflected from the half mirror 11 is to be the first focus detecting system A and another optical system lying on the optical path reflected from the mirror 11' is to be the second focus detecting system B. In this case, the amounts of defocus to be detected in terms of the focus detecting systems A and B are calculated from Equations (3) and (4) mentioned above.
Even in the case where the light receiving element arrays 14 and 17 are arranged on the same plane, the focus detecting device, unlike the prior art device early described (JP 63-88511), is designed to be able to set arbitrarily the spaces between the condenser lens 5 and the reimaging lenses 13 and 16, and between the reimaging lenses 13 and 16 and the light receiving element arrays 14 and 17, respectively, so that the imaging magnifications .beta..sub.1 and .beta..sub.2 can be made different from each other.
However, in the type of focus detecting devices such as JP 63-88511, the magnifications of the first and second focus detecting systems A and B need to be made equal and hence, must be intended for the extension of the focus detecting area and the improvement of the focusing accuracy according to the difference between the F numbers F.sub.W1 and F.sub.W2 of the barycentric beams. If the difference between the F numbers F.sub.W1 and F.sub.W2 is largely set, the F number of the photographic lens will be excessively small which is detectable in the focus detecting system intended for the improvement of the focusing accuracy, of the two focus detecting systems, and the photographic lens will be strictly limited in application. Alternatively, it is necessary to reduce the sizes of the apertures of the aperture stops in the focus detecting systems, with the resultant defect that a sufficient amount of light is not introduced to the light receiving element arrays and focus detection becomes difficult or impossible with respect to the object of low luminance.
Judging from the constructions the inventor worked out as shown in FIGS. 10 and 11, if the directions of the light receiving element arrays 14 and 17 of the focus detecting systems A and B are different, focus detection can be performed irrespective of the directions of contrast of the object. But the inventor found out that this constructions had defects as stated below.
Although the preceding problem is obviated because the difference between the magnifications is brought about by the two focus detecting systems, the images of the two focus detecting systems are formed farther away from each other and hence the area required for all the light receiving elements increases. Consequently, the defects are developed that the favorability of the light receiving elements is reduced and oversizing of the entire focus detecting system is brought about to adversely affect the compactness of the camera. Further, the mechanism is complicated and the assembly is incomplete, since adjustments are required for the half mirror 11 splitting the optical path into two focus detecting systems A and B and for the focus detecting systems A and B.